SEP 13, 2015 12:28 PM PDT

Newly Identified Group of Salmonella are Highly Virulent and Multi-Drug Resistant

Salmonella enterica are a group of rod shaped bacteria that are most commonly associated with foodborne illness in humans and animals caused by consumption of contaminated food and water. There are more than 2500 serotypes including nontyphoidal as well as Typhi and Paratyphi serotypes. Salmonella Typhi causes typhoid fever which results in serious, often fatal disease. Unlike nontyphoidal serotypes, S. Typhi can only infect humans. The story of Typhoid Mary describes the first “healthy carrier” of typhoid fever in the US. S. Typhi infections are rare in the US and occur more commonly in developing countries. Nontyphoidal serotypes are a leading cause of bacterial diarrheal illness worldwide.
Mary Mallon, also known as Typhoid Mary, was a cook in the early 1900s who infected over 50 people with S. Typhi.
Nontyphoidal Salmonella serotypes, such as S. Enteritidis, do not cause as severe disease as S. Typhi and are normally not fatal. The disease is characterized by diarrhea, abdominal cramps, vomiting and nausea and typically lasts up to 7 days in healthy individuals. In immunocompromised individuals, the disease can be fatal if not treated with antibiotics. Salmonella passes into the stomach where it binds to receptors on the lining of the intestine and is taken up by the cells. It is able to evade the host immune system by using our own cells to travel to other parts of the body such as the liver or spleen. The bacteria will replicate in the liver and released back into the intestine and released into the environment through feces. 
Showing the difference between Typhoidal, nonTyphoidal and invasive nonTyphoidal Salmonella
Researchers from Arizona State University describe a unique group of nontyphoidal Salmonella, known as Salmonella Typhimurium ST313, which has emerged as a fatal cause of bacteremia in young children as well as HIV-infected adults in sub-Saharan Africa. These isolates appear to have originated from a multidrug resistant clinical isolate of S. Typhi. While S. Typhi isolates are only able to infect humans, it appears the ST131 isolates have undergone a genome reduction that also allows them to infect non-human hosts. In their experiments, they demonstrated that one isolate from the ST131 group was able to cause lethal and invasive disease in mice where the bacteria was found to colonize the spleen, lymph nodes and gall bladder. The isolate also displayed resistance to low levels of pH, indicating its ability to survive passage through the acidic environment of the stomach.

The authors of this study warn that this isolate, technically identified as a S. Typhimurium ST313, may be evolving towards becoming a more human-specific “typhoid like” pathogen. ST313 infections do not result in symptoms typically associated with gastroenteritis. These isolates are a number one cause of sepsis in sub-Saharan Africa, which is often fatal. Treatment of these infections is also challenging because these isolates are often resistant to more than one type of antibiotic; therefore new treatment options are needed. This study, with the use of the mouse animal model, provides insight into how ST313 has evolved as well as how these isolates may cause disease in humans. More research needs to be done in order to develop better prevention and treatment strategies for those affected, specifically in developing countries.

Sources: Food Safety News, PLOS Neglected Tropical Diseases, Centers for Disease Control and Prevention, Salmonella.org, About Education
About the Author
  • I am a postdoctoral researcher with interests in pre-harvest microbial food safety, nonthermal food processing technologies, zoonotic pathogens, and plant-microbe interactions. My current research projects involve the optimization of novel food processing technologies to reduce the number of foodborne pathogens on fresh produce. I am a food geek!
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